Voltage ramp generators constitute basic elements of multiple electronic circuits and in particular in switched mode power supply circuits and choppers.
FIG. 1 illustrates the operating principle of a DC-DC converter—sometimes also called a switched mode power supply. Because of their high efficiency, such converters have become very popular and they are very frequently found in battery powered portable electronics. The voltage Vsense that is delivered at the connection terminals of a load resistor 3 results from a current, controlled by two transistors 5 and 6, and filtered by an LC network 1 and 2. A recovery diode 4 drains the energy stored in inductive resistor 1 when transistors 5 and 6 are off. Generally, the transistors are controlled by a comparator (not shown) that receives a reference voltage on a first input and a ramp voltage on a second input, as shown in FIG. 2. By varying the reference voltage Vref, the duty cycle between the current carrying time and the blocking period of transistor 5 is varied and, thus, voltage Vsense on the connection terminals of resistor 3 is changed.
In modern electronic devices, power supply circuits must be able to provide currents that can vary in a broad range of values. For example, mobile telephones have supply currents that can vary from several hundred of milliamps (when the apparatus is operating) to only a few microamps (in suspend mode).
Such variation ranges impose difficult operating conditions for power supply circuits and in particular, the ramp generator must allow regulation on the whole ramp range, including at both ends that correspond to the previously mentioned extreme values.
However, known ramp generators cannot really operate in the whole operating range considered because of the dispersion of components' characteristics, which results in dispersion on the slope—as shown in FIG. 3—and this makes it difficult to precisely control supply current near the ends. With known circuits it is not rare to observe a 30% rate of dispersion on the slope of the ramp generator.
It is thus desirable to have a ramp generator that provides a very well regulated slope that hardly depends on dispersions of component characteristics. Thus, precise control of the supply current is ensured between voltages ranging from a few microamps to several hundred milliamps and, more generally, if the slope is very well regulated, the constraints on the other components of the electronic circuit can be slackened.